The commercial scale production of detergent range (C9-C15 side chain) linear alkylbenzene (LAB) by a process which includes a step in which benzene is alkylated by linear monoolefins (MO) in the presence of hydrofluoric acid (HF) is known. In such a process, a mixture containing MO is contacted with excess benzene under suitable alkylation conditions to form LAB. Most of the HF is allowed to settle out as a separate HF rich liquid phase which is removed for recycle, and the remaining hydrocarbon rich liquid phase is then subjected to a series of fractionation (distillation) steps, optionally accompanied by one or more additional purification steps, in order to recover LAB of acceptable purity and also recover various species such as unconverted benzene and further amounts of HF for recycle. As used herein, the term "HF-LAB process" refers to a process of the type described above. It is also known to employ in an HF-LAB process a mixture containing MO produced by the dehydrogenation of the corresponding normal paraffins (NP). Such a mixture is produced by contacting a mixture containing NP under dehydrogenation conditions with a suitable catalyst such as platinum, resulting in partial conversion of the NP to MO. The resulting mixture is optionally concentrated and/or purified by one or more means such as distillation, selective hydrogenation, selective adsorption, etc. prior to use in the alkylation step. As used herein, the term "Dehy-HF process" refers to an HF-LAB process in which such a dehydrogenation mixture is used as the MO source. Usually in a process of this type, the mixture contacted with benzene contains both MO and a substantial amount of unconverted NP, in which case a distillation step is included to recover unconverted NP for recycle.
It is known that hydrocarbon mixtures as initially produced by any HF catalyzed alkylation process are usually contaminated by trace amounts of organic fluoride impurities (RF). The RF often includes species covering wide ranges of thermal stability and boiling point. Thus, as such a mixture is fractionated, objectionable amounts of both RF and HF formed by the thermal or catalytic decomposition of RF tend to be present in many if not all locations of the process downstream of the alkylation step, possibly including various recycle streams. A known method of greatly reducing this widespread migration of RF and widespread generation of HF is to include in the process at one or more locations a step herein referred to as "alumina treatment". In such a step, after separating most or all of the HF initially present in the alkylation mixture, either the entire remaining reaction mixture or one or more fractions of it are contacted with alumina under conditions sufficient to result in removal of most of the RF and HF. Such a method is described for example in U.S. Pat. No. 2,347,945.
In the production of LAB by the Dehy-HF process without the use of some method of RF removal such as alumina treatment, widespread migration of RF occurs to an extent sufficient to result in a number of significant disadvantages. One such disadvantage is fluoride contamination of the dehydrogenation catalyst resulting from its contact with RF or HF contained in recycled NP. Typically, the dehydrogenation catalyst includes an alumina support, and as such contamination increases, a point is soon reached such that the resulting acidic sites on the alumina support catalyze side reactions such as hydrocarbon cracking and isomerization to an objectionable extent. At this point one must choose between toleration of these side reactions or replacement of the dehydrogenation catalyst at considerable expense. Other disadvantages of widespread RF migration are associated with the resulting widespread breakdown of RF with the release of HF. Such disadvantages include increased corrosive attack upon processing equipment, the need to use more costly corrosion resistant materials of construction to keep such corrosive attack within acceptable limits, contamination of product and byproduct streams with HF, and increased risk of exposure of plant operating and maintenance personnel to HF.
In order to reduce the problems associated with widespread RF migration in the production of LAB by the Dehy-HF process, one or more alumina treatment steps are typically included. For purposes of minimizing such problems, the most advantageous approach is to employ a single alumina treatment step located immediately after the separation of the HF from the alkylation mixture. For example, if a single distillation column is used to recover both the dissolved HF and the unconverted benzene, an alumina treater including one or more stages can be located in the stream recovered from the bottom of this column. Such location minimizes the adverse effects of RF in all downstream equipment involved in the fractionation and purification of the alkylation mixture. Since such downstream equipment includes the paraffin recovery column, RF is also greatly reduced in the recycle paraffin stream, and this protects the dehydrogenation catalyst from excessive exposure to RF.
However, in previously known Dehy-HF processes, it has been found that exposure to the alumina treatment step of the alkylation mixture fraction having a higher boiling range than the unconverted benzene has resulted in a large adverse effect upon the quality of the LAB produced. This adverse effect is most apparent when such LAB is sulfonated using SO3 (as distinguished from oleum) in the production of detergents. The sulfonated product contains such a high level of darkly colored materials that it is difficult to use it to produce detergent formulations of acceptable appearance. As used herein, the term "SO3 sulfonation color" refers to the degree to which the LAB has the tendency to produce such darkly colored SO3 sulfonation products.
It has been found that if only the recycled NP stream is subjected to alumina treatment, adverse effects of alumina treatment upon the quality of the LAB can be largely avoided. Production of LAB by a Dehy-HF process in which the alumina treatment step is located in the recycle paraffin stream is widely practiced (B. Vora et al, Chemistry & Industry, 19 Mar., 1990, pp. 187-191). Such processes are known to be capable of producing LAB of good quality, but they suffer from the disadvantages of allowing higher levels of RF and HF within the paraffin recovery column and all downstream equipment. Such downstream equipment must be fabricated to withstand the highly corrosive effects of HF resulting in higher capital costs, and the presence of HF within this equipment results in greater risk of exposure of plant workers to HF.